Light weight driveshafts and alloys therefor are disclosed in U.S. Pat. Nos. 4,392,839; 4,527,978; 5,320,580; 5,607,524; and 5,637,042. Such driveshafts can be made by various techniques such as machining a solid billet, extruding a solid or hollow billet, seam welding, etc. According to the '524 patent, alloy 6061 has been used for driveshaft applications, the alloy including 0.8-1.2% Mg, 0.4-0.8% Si, 0.15-0.4% Cu, 0.04-0.35% Cr, the balance being Al and incidental impurities. Depending on the type of vehicle, aluminum driveshafts can range from 2 or 3 inches in outside diameter to 4 to 5 inches or even higher, e.g. 6 to 7 inches or higher. Typical wall thicknesses can be 0.05 or 0.06 inch up to about 0.08 or 0.09 inch or even thicker. An example of a driveshaft for an automobile could have an outside diameter of about 3.5 inches and a wall thickness of about 0.08 inch whereas a driveshaft for a truck could have an outside diameter of 4.5 to 5 inches and a wall thickness of 0.07 to 0.09 inch. In order to transmit power, each end of the driveshaft typically includes a yoke welded thereto. The yokes cooperate with universal joints to transmit power from a rotating member to a member to be rotated such as a wheel.
Aluminum-magnesium-silicon extrusion alloys are disclosed in U.S. Pat. Nos. 4,113,472; 4,231,817; 4,256,488; 5,223,050 and 5,240,519. Of these, the '472 patent discloses an alloy including 0.9-1.5% Mg, 0.4-0.8% Si, and 0.9-1.5% Cu. The '817 patent discloses an alloy including 0.45-0.98% Mg, 0.3-0.8% Si and 0.5-0.25% Zn, the alloy optionally including 0.04-0.30% Cu, 0.04-0.25% Zr, 0.04-0.30% Cr and 0.04-0.25% Mn and possibly 0.01-0.20% Ti and/or 0.01-0.06% B. The '488 patent discloses an alloy including 0.030-0.60% Mg, 0.45-0.70% Si and 0.10-0.30% Cu with optional inclusions of up to 0.35% Fe, up to 0.15% Zn and up to 0.010% Ti. The '050 patent discloses 6000 series alloys including 6082, 6351, 6061 and 6063 wherein Mg is present as Mg.sub.2 Si .beta.'-phase particles. The '519 patent discloses an alloy having 1.0-1.5% Si, 0.4-0.9% Cu, 0.2-0.6% Mn, 0.8-1.5% Mg, 0.3-0.9% Cr, 0.03-0.05% Ti, 0.0001-0.01% B, balance Al and impurities.
Commercial automotive drawn 6061 tubing for driveshaft assemblies having yokes welded thereto is produced in T8 and T6 tempers. The T6 temper is produced using a furnace solution heat treatment after cold drawing to finish dimensions. Such solution heat treatments are conducted close to the melting point of the aluminum alloy and thus necessitate use of sophisticated equipment and process control, making the T6 process relatively expensive to carry out. The T8 temper is produced by solutionizing as part of the extrusion process, followed by drawing to finish dimensions. However, when the 6061 alloy is extruded into seamless tubing, grain size variation may occur from surface to center of the tube wall and there may be grain size variation along the length of the tube. Such variation can cause wrinkling during end reduction (a process used to adapt the tube ends to their associated yokes) and excessive total indicated runout ("TIR") on reduced end tubes.
There is a need in the art for an economical process which produces light weight extruded aluminum tubes which can be used in driveshaft assemblies. It would be desirable for the aluminum alloy to exhibit a uniform fine grained structure which allows such extruded tube to be drawn without the need for an expensive separate furnace heat treatment. It would also be desirable if the alloy composition would allow the load requirements for extrusion to be reduced and thus increase the extrusion press productivity. Moreover, it would be desirable if such objectives could be met while also providing increased strength in the heat affected zone adjacent to the weld in the driveshaft assemblies while maintaining sufficient mechanical strength and torsional fatigue resistance.